The present invention relates to a glow plug for preheating a subcombustion or combustion chamber of a diesel engine and, more particularly, to an improvement of a diesel engine glow plug of a bipolar two-line system having a ceramic heater which has high-speed and self temperature saturation properties and which allows "after glow" operation for a long period of time.
Conventional glow plugs having various types of structures have been proposed. Among these glow plugs, a plug having a ceramic heater has received a great deal of attention as a fast heating plug.
A glow plug of a ceramic heater type is described in Japanese Patent Prepublication No. 60-14784. This glow plug has a structure wherein a heating element is exposed on the outer surface of a heater by using a conductive ceramic material having substantially the same thermal expansion coefficient as that of an insulating ceramic material with a heater insulating element, so that the heating element is integrally formed with the heater insulating element. With this structure, the distal end of the heater can be immediately heated to obtain a fast heating glow plug. At the same time, bonding between the heating element and the heater insulating element can be optimally and appropriately maintained to improve reliability for heat resistance and the like to some extent.
In a conventional glow plug of a ceramic heater type having the structure as described above, however, many problems are left unsolved from structural and functional points of view when such a plug is used as a glow plug in an operating engine.
With the above structure, the heating element is exposed on the outer surface of the heater to achieve fast heating. However, the heating element is formed by a simple U-shaped laminated structure, and both ends thereof are guided to the rear end portion of the heater. In order to produce a practical glow plug, an electrode extraction portion and a holding portion to be coupled to a holder must be specifically designed. For example, when external connection electrodes are led out from the ceramic heater, the lead portion must be separated from the heating element at the distal end of the heater by as great a distance as possible to minimize a thermal influence, thereby improving reliability for bonding strength or the like. The thermal influence must also be considered at a bonding portion held by the holder by silver brazing. The heat resistance of the bonding portion must be assured. It is very difficult to satisfy these requirements.
In the conventional ceramic heater as described above, the heating element has an integral structure of conductive and insulating ceramic materials. Although the thermal expansion coefficients of these ceramic materials are substantially the same, reliability of the bonding portion of a ceramic heater in a glow plug heated to 1,100.degree. C. or more is undesirably low.
In a glow plug of this type, smooth and efficient combustion inside the engine can be achieved due to improvements in the starting characteristics of the diesel engine; durability for high-temperature operating conditions as a result of the widespread use of turbo mechanisms and; maintenance of an energized state for the glow plug for a predetermined period of time after starting the engine. This better combustion results in a reduction of exhaust gas and noise. Market demand has arisen for such an after glow system, and maximum prolongation of the after glow time (e.g., 10 minutes) is required. In order to prolong the after glow time, energization power to the heating element must be self-controlled to greatly improve the heating characteristics. Overheating of the heater portion must be prevented, and a self temperature saturation function is required to keep the saturation temperature below an appropriate temperature. During after glow operation, a voltage applied to the glow plug is kept lower than that applied at the time of energization of the plug so as to assure durability of the heating wire. In consideration of these points, a demand has arisen for development of a low-cost glow plug having a ceramic heater having fast heating and self temperature saturation properties as well as high reliability for heat resistance and the like.